1. Field of the Invention
This invention relates to a fluorescent substance and a light-emitting device using the same, more particularly to a fluorescent substance that has an yttrium aluminum garnet (YAG) structure and an emission spectrum having a dominant wavelength ranging from 500 nm to 550 nm.
2. Description of the Related Art
In recent years, light emitting diodes (LED) have been widely applied and serve as a light source in various devices such as fluorescent light, vehicle lighting equipment, display board, and back light for a liquid crystal display, etc. LEDs have the advantages of high light emission efficiency, small size, low power consumption, etc. Moreover, manufacturing cost of the LED is relatively low.
At present, there are two primary methods for producing white light LEDs. One is to use individual LEDs that respectively emit red, green, and blue lights, and then to mix the lights, thereby forming white light. The other is to use a phosphor or fluorescent material to convert monochromatic light emitted from an original LED (mostly blue or ultraviolet LED), followed by mixing the light from the original LED and from the fluorescent material to produce light that appears white. Depending on the color of the original LED, the fluorescent substance that emits light of different colors may be adjusted. For the second method, the original LED is usually a blue LED. Fluorescent substances that emit yellow light or red and green light after being excited by the blue LED are mostly used accordingly.
Although the LEDs produced from the above second method may emit high intensity white light, there is a problem of lower emission efficiency than normal LEDs due to heat loss from the Stokes shift and other degradation issues. Particularly, a blue LED combined with yellow-emitting fluorescent material has inferior luminance and color saturation, and thus, cannot be used as a back light source. For a blue LED combined with red-emitting and green-emitting fluorescent materials, the green-emitting fluorescent material has problems of low luminance and insufficient durability. For example, a silicate fluorescent material is easily deteriorated because of its hygroscopic property; and a nitrogen series fluorescent material usually has inferior luminance.
Japanese Publication No. 2000-219879 discloses a fluorescent substance used for a field emission display (FED). The fluorescent substance includes alkali metals and alkaline-earth metals and a composition represented by Ln3(Al1-xGax)5O12:Re, wherein Ln is yttrium(Y), lanthanum(La), or gadolinium(Gd), Re is terbium(Tb), cerium(Ce), or europium(Eu), and 0≦x≦1. The content of the alkali metals and the alkaline-earth metals is lower than 5 ppm. By controlling the content of the impurities, i.e., the alkali metals and the alkaline-earth metals, the luminance and durability of the fluorescent substance can be improved. Nevertheless, the Japanese publication does not discuss the relation between the ratio of metal elements of the composition of the fluorescent substance and the luminance thereof. Moreover, control of all of the alkali metals and the alkaline-earth metals is somewhat complicated. In addition, the fluorescent substance is produced by co-precipitation without using flux, which has low production efficiency, and thus is inapplicable for industrial mass production.
Therefore, it is desirable in the art to provide a fluorescent substance that has the properties of high luminance, color saturation and long durability, as well as being applicable for industrial production.